Process of producing ammoniated superphosphates



United. States Patent 4 PROCESS OF PRODUCING AMMONIATE SUPERPHOSPHATESRichard C. Datin, Petersburg, Va., assignor to Allied Chemical & DyeCorporation, New York, N. Y., a corporation of New York No Drawing.Application May 13, 1953, Serial No. 354,881

13 Claims. (Cl. 71- -37) This invention relates to the production offertilizers, and more particularly to the production of ammoniatedsuperphosphates.

The ammoniation of superphosphates employing about 1.3 mols of ammoniaor less per mol ,of P205 to convert part of the water-solublemono-calcium acidphosphate to citrate-soluble phosphates is well known.Use of larger amounts of ammonia, to introduce additional nitrogen,conveniently and inexpensively supplied by ammonia, results in reversionof water-soluble and citratesoluble phosphates in which form it isavailable as plant food, to a citrate-insoluble form, i. e., the formsof P205 unavailable as plant food and insoluble in ammonium citratesolutions as determined by the procedure of analysis of the Associationof Ofiicial Agricultural Chemists (Methods of Analysis, 7th ed.,Washington,

1950). It has been found that the amount of P205 reversion to thecitrate-insoluble form tends to increase with :an increase in thequantity of ammonia added to the superphosphates.

While the causes for reversion to citrate-insoluble P205 are not known,it is believed by many researchers in this field that it is dueto'reaction of tricalcium phosphate and calcium fluoride present inhighly ammoniated superphosphate to form fluorapatite (3Cas(P04)2CaF2).Phosphate rocks generally used in making ammoniated superphosphatecontain from 2 to 4.5% of fluorine which is ample to cause reversion. 7

It is among the objects of this invention to'provide a process ofproducing ammoniated superphosphates containing citrate soluble P205which has little or substantially no tendency to revert to thecitrate-insoluble form and-this notwithstanding that substantially inexcess of 1.3 mols of ammonia are employed per mol of P205 in theammoniation of the superphosphatel. 7

Other objects and advantages of this invention will be apparent from thefollowing detailed description thereof.

In accordance with this invention the acidulation of the phosphate rockis carried out in the presence of sodium ion, desirably from 0.25% to10% sodium and preferably from 1.0% to 5% sodium based on the weight ofsolids in the superphosphate. Surprisingly, it has been found that bycarrying out the acidulation of the phosphaterock in the presence ofsodium ion, .and thereafter ammoniating the supcrphosphate thusproduced, employing a much greater amount of ammonia than 1.3 mols ofammonia per mol of P205, for example 1.6 mols of ammonia per mol of P205and even more an ammoniated superphosphate results in which there ismaterially less tendency for reversion of citrate-soluble P205 to thecitrate-insoluble form to take place.

One possible explanation for the unexpected behavior of the presence ofthe sodium ion during acidulation to materially minimize reversion ofP205 in'the ammoniated product to the citrate-insoluble form is that thepresence oflthe sodium blocks the normal formation of fluorapatiteand/or-results in the formation ofapresently unknown complex P205compound containing sodium having a telatively high solubility inammonium citrate solution, and, hence, high P205 availability as a plantnutrient. This explanation is advanced in the interests of facilitatinga better understanding of the invention, and with the understanding thatthis invention is not to be limited thereby.

The sodium ion is introduced just prior to or during the acidulation inthe form of asodium compound, e. g., a sodium salt such as sodiumnitrate, sodium chloride, sodium sulfate, sodium phosphate, sodiumborate, so-

dium carbonate, or other sodium salt including sodium fluoride, whichhowever is somewhat less effective in decreasing phosphate reversionthan most of the other sodium salts above enumerated. Should sodium beadded as such or as hydroxide it would react with the acid employed toeffect the acidulation, to form the salt of the acid. Hence, sincesodium or its hydroxide is more expensive and moredifiicult to handlethan the salt, it is preferred to use the salt. The sodium ion may beadmixed with the acid and the resultant acid salt mixture added to thephosphate rock to effect the acidulation, or it may be added to thephosphate rock before or during the addition of the acid thereto. Theimportant factor is that the sodium be present during the acidulation.The nature of the ion associated with the sodium appears relativelyunimportant; all thus far tested reduce the tendency for reversion ofcitrate-soluble P205 to the citrateinsoluble form to take place althoughsome ions, e. g., sulfate, nitrate and chloride appear to do so betterthan others, e. g., the fluoride.

Two procedures are now commonly used for producing ammoniatedsuperphosphate and this invention may be employed in connection withboth to reduce the tendency for reversion of citrate-soluble P205 to thecitrateinsoluble form to take place. In one process, only a small excessof acid, usually sulfuric acid, is used to acidulate the phosphate rockto form solid superphosphate which is ammoniated after curing. By excessacidv in this specification is meant the amount of. acid over and abovethe amount required to solubilize the P205 in the rock. In thisprocedure in lieu .of sulfuric acid, mixed acids may be used, such asmixtures of sulfuric and pitric acids, sulfuric and phosphoric acids, ornitric and phosphoric acids. In general, in this procedure at least 3and usually about'5 equivalents of acidper mol'of P205 are employed toeffect the acidulation. By an equivalent of acid is meant the quantityof acid which contains one formula weight of hydrogen ion, i. .e., ahalf mol of H2804, one mol of HNOs and one mol .of H3PO4. For purposesof determining the equivalents of acid, in the processes embodying thisinvention, phosphoric acid is considered a monobasic acid, becausemono-calcium phosphate or CaH4(P04)2 comprises-the major portion ofthephosphoric acid in superphosphates, i. e., two hydrogen ions remainwith the phosphoric acid.

In the other procedure, Which will be hereinafter referred to-as theslurry procedure, the phosphate rock is treated with a considerableexcess of acid forming a slurry which is ammoniated immediately afterits formation and the ammoniated superphosphate is then dried.

' The acid used may bea mixture of sulfuric and nitric used may be from6 to 20, preferably from 6 to- 15,

equivalents of-acid perniol of P205 in the phosphate Patented Fe'b. 26,"1957* rock. In the slurry procedure, it is that potrion of the ammoniaover and above that which is required to neutralize the excess acid,that is responsible for reversion of citrate-soluble phosphates to acitrate-insoluble form and whichaction is inhibited by the presentinvention.

As above noted, in the slurry procedure from 6 to 20, preferably from 6to 15, equivalents of acid per mol of P205 in the phosphate rock areused in the acidulation step. In the other procedure hereinabovedescribed, at least 3 and usually about 5 equivalents of acid areemployed per mol of P205. Hence, in the acidulation step of thisinvention considered generically, i. e., as including both procedures,the phosphate rock may be treated with from 3 to 20, desirably 3 to 15,equivalents of acid per mol of P20 in the phosphate rock.

The rock and acid are desirably introduced at room temperature. Theacidulation is an exothermic reaction and may be carried out at anydesired temperature below that at which substantial loss of nitrogencompound takes place. The acidulation may be carried out withoutexternal cooling. By introducing the Dock and acid at atmospherictemperature, the reaction mixture in general will not go above about 85to 100 C. Preferably, thereaction mixture is left at this temperatureuntil the acidulation reaction has been completed. The acidulationreaction is carried out in the presence of from about 15% to about 45%water based on the total weight of the constituents present in theacidulator.

The acidulated mixture, referred to hereinafter as superphosphate, isammoniated employing from about 1.6 to 2.9 mols of ammonia per mol ofP205 in the superphosphate. This range, refers to the range of amountsof am monia employed in the ammoniation over and above the amount ofammonia added to neutralize the excess acid. Thus, in the firstprocedure hereinabove disclosed, involving the use of about 5equivalents of acid per mol of P205 (which amount represents little ifany excess acid) from about 1.6 to 2.9 mols of ammonia per mol of P205in the superphosphate is, employed in the ammoniation. In the slurryprocedure the amount of animonia used is greater by an amount equal tothat required to neutralize the excess acid. The use of 1.6 mols ofammonia per mol of P205 (over and above the amount required toneutralize the excess acid) represents an increase of about 23% over andabove the amount of ammonia heretofore used and which was generallyconsidered the maximum permissible in order to avoid excessive reversionof P205 to the citrate-insoluble form.

The ammoniation is carried out in the presence of water, usually waterin amount of from 7% to 10% of the ammoniated mixture will be foundadequate, although larger amounts of water, as high as 50%, may bepresent, for example, when using 42% nitric acid or weakeracid to effectthe acidulation. phosphate leaving the ammoniator generally containsless water than the superphosphate ammonia reaction mixture.

Aqueous ammonia solutions or gaseous ammonia may be employed to eifectthe ammoniation. Also solutions of salts having fertilizer value, forexample, ammonia solutions of ammonium nitrate, or of sodium nitrate,and such solutions containing urea may be employed to effect theammoniation.

The ammoniated superphosphate mixture may be subjected to a granulation,or drying treatment, if desired, to produce the desired product. Asabove noted this invention includes processes in which the acidulationis carried out in the presence of sodium ion, the superphosphate thusproduced is ammoniated, and the ammoniated superphosphate is usedwithout further treatment, or is subjected to a drying or granulationtreatment or is admixed with a potassium salt or other plant nutrientwith or without additional treatment such as granulation of theresulting mixture.

The following examples are given for purposes of illustrating theinvention; it will be understood the invention The ammoniatcd superisnot limited to these examples. parts are by weight.

EXAMPLE LfiSODIUM NITRATE PRESENT DUR- ING THE ACIDULATION 15 parts of60% H2504 at a temperature of C. were added to a mixture consisting of15 parts phosphate rock, 2.80 parts sodium nitrate and 0.20 part sand,the mixture agitated for 1 minute and then maintained at 85 C. for 20hours.

Three additional batches were prepared by the same procedure, thesebatches, however, containing 2.01 parts sodium nitrate and .99 partsand, 1.59 parts sodium nitrate and 1.41 parts sand, and 0.97 part ofsodium nitrate and 2.03 parts sand, instead of the 2.80 parts sodiumnitrate and 0.2 part sand of the first-mentioned batch. Thus each batchconsisted of thirty-three parts; the sodium nitrate and sand content ofall batches consisted of 3 parts but the relative amounts of sand andsodium nitrate from batch to batch differed as noted above.

For purposes of comparison a fifth batch was prepared in the samemanner, except that no sodium nitrate was used and 3 parts of sand weremixed with the 15 parts of phosphate rock subjected to acidulation.

Like portions of each batch were ammoniated employing a solutioncontaining 21.7% ammonia, 65% ammo nium nitrate and 13.3% water. 0.29pound of this solution was used per pound of superphosphate. Theammoniation was carried out at atmospheric pressure, with a watercontent of 10% of the total mixture and took /2 minute to spray theammonia solution on each batch followed by mixing for about 5 minutes.Thereafter each batch was stored at 80 C. for 30 days. The batches werethen analyzed to determine their total P205 and citrateinsoluble P205.These analyses were carried out by regular Association of OfficialAgricultural Chemists procedures. The results are given in the tablewhich follows:

Table I In these examples all Percent Na (added as (added as NaNOg)NaNOa) present during present in the acidulation nmmoniated productPercent Na Pcrccntunavailable P205 X citrateinsoluble P101. 150ml P305)Comparative batch.-. Batch No. 1 Batch N0. 2-. Batch N0. 3-- Batch N0. 4

EXAMPLE II.-SODIUM SULFATE PRESENT DURING THE ACIDULATION Table IIPercent N it (added as N aaSOt) present during acidulation Percent Na(added as NazSQl) present in the ammoniated product Percent unavailablePaOl (100 X citrateinsoluble P total Comparative batch.. Batch N 5 BatchNo. 7

EXAMPLE IIL-SODIUM I CHLORIDE PRESENT DURING THE ACIDULATION Thetechnique followed was the same as in the other examples, except thatsodium chloride was the salt employed to furnish sodium ion in theamounts indicated in Table III which gives the results of the analysesof the batches subject to test:

' Table III Percent unavailable P205 (IOOXcitrate- N 2101) presentinsoluble in the ammo- P2O5+total mated product P20 Percent N a (addedas Percent Na (added as N 2.01) present during acidulation Com arativebatch... Bate No. 8 Batch No.9 Batch No. 10

Table IV Percent unavailable P202 (IOOXcitrateinsoluble P2O +total P2 5)Percent Na (added as N aF) present in the ammoniated product Percent Na(added as NaF) present during acidulation Comparative batch Batch No. 11

10.75 4.4 Batch N0.12.. 2 8

EXAMPLE V.-SODIUM NITRATE PRESENT DUR- ING THE ACIDULATION WITH MIXEDACID Two superphosphates (mix 1 and mix 2, respectively) were made asfollows:

For mix 1, 23.5 pounds of 42% nitric acid and 5.5 pounds of 94% sulfuricacid were added to 21 pounds of phosphate rock. For mix 2, 23 pounds of42% nitric acid containing 1 pound of dissolved sodium nitrate and 5.4pounds of 94% sulfuric acid were added to 20.6 pounds of phosphate rock.

A portion of each mix was ammoniated with a solution containing 21.7%ammonia, 65% ammonium nitrate and 13.3% Water. Approximately 8.24 poundsof solution were used per pound of the superphosphate. The products werestored at 50 C. for 30 days with a moisture content of approximately11%. The ammoniated products were analyzed for citrate-insolub lesbefore and after storage, and its was found that the ammoniated productfrom mix 2 containing sodium added during the acidulation showedmaterially less reversion in storage than the ammoniated product frommix 1 which did not contain any added sodium.

From the above examples it will be noted the presence of the sodium ionduring the acidulation results in a very material reduction in theformation of citrate-insoluble P205 as compared with acidulationprocedure followed by subsequent ammoniation under the same conditions,except for the omission of the sodium ion during the acidu lation.

It is an important feature of this invention that the sodium ion bepresent during the acidulation, i. e., added before the acidulation ofthe phosphate rock has been completed. The incorporation of the sodiumion during the ammoniation does not give the same results. In a seriesof comparative tests involving the incorporation of the same quantity ofsodium nitrate (1.1% Na based on the weight of ammoniated product) itwasfound that-the] incorporation of the sodium during" the ammoniat ionresulted in a decrease of only 23% in the amount of citrateinsolubleP205 in the ammoniated product after storage for 30 days at (3., whereasthe incorporation of the same amount of sodium ion during theacidulation re: sulted in a decrease of 92% under the same testconditions.

Since certain changes may be made in carrying out the above processwithout departing from the scope of the invention, it is intended thatall matter containedin the above description shall be interpreted asillustrative and not in a limiting sense.

What is claimed is: v 4

1. The process of producing ammoniated superphosphate which comprisesacidulating phosphate rock con taining fluorine in the presence of addedsodium ion and thereafter ammoniating the superphosphate producedby theacidulation employing at least 1.6 mols of ammonia per mol of P205 overand above the amount of ammonia required to neutralize that portion ofthe acid employed during the acidulation which is in excess ,overthatrequired to solubilize the P205 in the phosphate rock, -2. The processdefined in claim 1 in which from 0.25 to 10% by weight of sodium, basedon the weight of solids in the superphosphate, is present during theacidulation.

3. The process defined in claim 1, in which from 1% to 5% by weight ofsodium, based on the weight of solids in the superphosphate, is presentduring the acidulation.

4. The process of producing ammoniated super-phosphate which comprisesacidulating phosphate rock containing fluorine employing from 3 to 20equivalents of acid per mol of P205 in the phosphate rock and in thepresence of from 0.25% to 10% by weight of sodium, based on the weightof solids in the super-phosphate, and ammoniating the super-phosphatethus produced employing at least 1.6 mols of ammonia per mol of P205over and above the amount of ammonia required to neutralize that portionof the acid employed during the acidulation which is in excess over thatrequired to solubilize the P205 in the phosphate rock.

5. The process of producing ammoniated super-phosphate which comprisesacidulating phosphate rock containing from 2% to 4.5% by weight offluorine employing from 3 to 20 equivalents of acid per mol of P205 inthe phosphate rock and in the presence of from 0.25% to 10% by weight ofsodium, based on the weight of solids in the super-phosphate, andammoniating the superphoph ate thus produced employing from about 1.6 toabout 2.9 mols of ammonia per mol of P205 over and above the amount ofammonia required to neutralize that portion of the acid employed duringthe acidulation which is in excess over that required to solubilize theP205 in the phosphate rock.

6. The process of producing ammoniated superphosphate which comprisesacidulating phosphate rock containing from 2% to 4.5% by weight offluorine employing from 3 to 15 equivalents of acid per mol of P205 inthe phosphate rock and in the presence of from 1% to 5% by weight ofsodium, based on the weight of solids in the superphosphate, andammoniating the superphosphate thus produced employing from about 1.6 toabout 2.9 mols of ammonia per mol of P205 over and above the amount ofammonia required to neutralize that portion of the acid employed duringthe acidulation which is in excess over that required to solubilize theP205 in the phosphate rock.

7. The process defined in claim 6, in which the sodium is present duringthe acidulation in the form of sodium nitrate.

8. The process defined in claim ,6, in which the sodium is presentduring the acidulation in the form of sodium chloride.

9. The process defined in claim 6, in which the sodium 7 is presentduring the *acidulation in the form of sodium fillfidfi:

10. Theprocessdefined in claim 6, in which the sodium is present duringthe acidulation in the form of sodium sulfate.

11. The process defined in claim 6, in which the sodium is presentduring the-acidulation in the form of sodium phosphate.

12. A process of producing ammoniated 'superphosphate which comprisestreating phosphate rock containing from 2% to 4.5% by weight of fluorinewith about 5' equivalents of sulfuric acid per mol of PzOs in the rockand in the presence of from 1% to 5% by weight of sodium based on theweight of solids in the superphosphate, said sodium being present as asodium salt, and ammoniating the superphosphate thus produced cmploying;from about 1.6 to about 2.9 mols of ammonia per mol of P205 in thephosphate rock.

13. A process of producing ammoniated super-phosphate which comprisestreating phosphate rock contain ing from 2% to 4.5% by Weight offluorine with from 6 to 15 equivalents of acid per mol of P205 in therock, and in the presence of from 1% to 5% by weight of sodium based onthe weight of solids in the superphosphate, said sodium being present asa sodium salt, said acid. consisting of a mixture of sulfuric and nitricacids containing sutiicient sulfuric acid to form calcium sulfate withall of the calcium present in the rock in excess of that required toform dicalcium phosphate with the P205 content of the rock andammoniating the supcrphosphate thus produced employing per mol of P205in the phosphate rock, from, about 1.6 to about 2.9 mols of ammonia overand above the amount of ammonia required to new. tralize that portion ofthe acid employed during the acidulation which is in excess, over thatrequired to solubilize the P205 in the phosphate rock.

References Cited in the file of this patent UNITED STATES PATENTS 49,831Liebig Sept. 5, 1865 252,029 Gibbons et al Jan. 10, 1882 1,351,672 MeigsAug. 21, 1920 1,637,423 Rupp Aug. 2, 1927 1,712,404 Rupp May 7, 19291,849,989 Moore Mar. 15, 1.932 1,929,710 Ober et a1. Oct. 10, 19331,948,520 Harvey Feb. 27, 1934 2,035,484 Kniskern et a1 Mar. 31, 1.9362,739,054 Andres et a1 Mar. 20, 1956 FOREIGN PATENTS 5,686 Great BritainApr. 27, 1886

1. THE PROCESS OF PRODUCING AMMONIATED SUPERPHOSPHATE WHICH COMPRISINGACIDULATING PHOSPHATE ROCK CONTAINING FLUORINE IN THE PRESENCE OF ADDEDSODIUM ION AND THEREAFTER AMMONIATEING THE SUPERHOSPHATE PRODUCED BY THEACIDULATION EMPLOYING AT LEAST 1.6 MOLS OF AMMONIA PER MOL OF P2O5 OVERAND ABOVE THE AMOUNT OF AMMONIA REQUIRED TO NEUTRALIZE THAT PORTION OFTHE ACID EMPLOYED DURING THE ACIDULATION WHICH IS IN EXCESS OVER THATREQUILRED TO SOLUBILIZE THE P2O5 IN THE PHOSPHATE ROCK.